Emission regulations are an important source for driving innovation in the development of cleaner running engines. The automotive industry has made many advances in engine design, engine controls, fuel injection and the like in order to improve emissions. The exhaust system has been of particular importance in meeting and exceeding regulations. In order to insure proper function of the exhaust system and engine overall, various sensors provide information to the engine controls. Typical sensors include temperature sensors, pressure sensors and oxygen sensors.
The exhaust system is a particularly difficult environment in which to deploy sensors. In both diesel and gasoline applications, internal combustion engine design is focused on increasing power density, fuel efficiency, and engine agility. These developments are driving the exhaust systems to higher temperatures and higher gas speeds together with sharper temperature changes. Typical operating temperatures range from the very cold ambient temperature at start up to 750-1200° C. during operation. The thermal shocks may be as much as 1100 Ks−1 and induce a significant amount of mechanical stress. Further, the exhaust system often has caustic gases that can be quite corrosive. Along with these rapid temperature changes and chemically hostile environment, significant vibration is not unusual. As a result, sensor life is often limited and failure can occur. In contrast, other components of the exhaust system of an internal combustion (IC) engine have a long lifetime.
Typically, high temperature sensors consist of a tip assembly which is exposed to the medium to be measured. With exhaust gas temperature sensors, the tip assembly may be in contact with the exhaust gas. A temperature sensing element is housed within the tip assembly so that heat-flux is transferred into the tip assembly. The internal temperature sensing element could be a positive temperature coefficient (PTC) thermistor or negative temperature coefficient (NTC) thermistor.
Several examples of exhaust gas sensor technology are U.S. Pat. No. 6,639,505 B2 issued Oct. 28, 2003 to Murata et al., U.S. Pat. No. 6,829,820 B2 issued Dec. 14, 2004 to Adachi et al., U.S. Pat. No. 8,328,419 B2 issued Dec. 11, 2012 to Wienand et al., U.S. Pat. No. 5,831,512 issued on Nov. 3, 1998 to Wienand et al., U.S. Pat. No. 8,333,506 issued on Dec. 18, 2012 to Kamenov et al., U.S. Pat. No. 6,617,956 issued on Sep. 9, 2003 to Zitzmann, and U.S. Pat. No. 6,353,381 issued on Mar. 5, 2002 to Dietmann et al., each of which is incorporated herein by reference.
A common positioning of the exhaust gas temperature (EGT) sensor in the outlet of an exhaust system creates high positive and negative temperature gradients on the tip as illustrated in FIG. 1. The gradients create a delta on the tip assembly 900 between the outer portion of the metal tube 902 and the inner portion the metal tube 902. The sensing element 904 is typically near a distal end of the metal tube 902 and embedded in a cement paste. Leads 906 carry the signal from the sensing element 904. The electrical leads 906 are mechanically coupled. A standard approach is to have mineral insulated cable with the leads in a fixed position.
The temperature delta creates positive and negative thermal shocks. Negative thermal shocks, illustrated by arrow 908, create pushing on the sensing element 904 as shown by arrow 910. Positive thermal shocks, illustrated by arrow 912, create pulling on the sensing element 904 as shown by arrow 914. The result is harsh compressive and tensile stresses between the internal components, connections and electrical leads 906. The stresses lead to damage and failure.